Distillation column operation method

Abstract

Provided is a technique for reducing the lower-limit value for the supply flow rate of a distillation column. This method for operating a distillation column includes: a step for adjusting a fluid being treated, which is supplied to the distillation column, to a turndown-operation-time supply flow rate that is lower than a design-operation supply flow rate; a step for increasing a turndown flux flow rate and a turndown reboiler flow rate, and raising a column-internal air-liquid load, in order to make the properties of a column-bottom product flowing out from the column bottom heavier than those in the design operation while satisfying specifications required in design, or making the properties of a column-top product flowing out from the column top heavier than those in the design operation while satisfying specifications required in design, for the distillation column to which the fluid being treated is supplied at the turndown-operation-time supply flow rate; and a step for verifying that the properties of the column-top product or the column-bottom product satisfy required specifications.

Description

Technical Field

[0001] The present invention relates to an operating method of a distillation column.

Background Art

[0002] When producing LNG (Liquefied Natural Gas) from natural gas (NG: Natural Gas), an LNG plant is used. This LNG plant includes a distillation column (scrub column) that performs a process of separating and recovering heavy hydrocarbons from NG to obtain light hydrocarbons mainly composed of methane as a gas treatment device after removing impurities from natural gas, and a distillation column (stabilizer) that further precisely distills and separates the heavy hydrocarbons after the light hydrocarbons are distilled and separated in the above-described scrub column (for example, FIG. 11 of Patent Document 1).

[0003] For each distillation column provided in the LNG plant, a lower limit value of the supply flow rate for stable operation is determined based on the mechanical structure factors of the distillation column (for example, internal structures such as column diameter, column height, trays, etc.). Therefore, if the supply flow rate of NG at startup is further reduced below the lower limit value based on the mechanical structure factors, neither the light hydrocarbon component (LNG) nor the heavy hydrocarbon component (condensate) will meet the specifications. As a result, until the supply flow rate of NG reaches the required minimum supply flow rate and meets the design requirements, it has been necessary to send these to the flare stack and continue burning them. For this reason, there has been a problem that an increase in the combustion amount of hydrocarbons leads to a further increase in the emission amount of CO2, which is a GHG (Greenhouse Gas). In addition, not limited to LNG plants, in distillation columns provided in other types of gas treatment devices, while the reduction of greenhouse gas emissions is required, for operation at a supply flow rate lower than the design lower limit value of the distillation column, equivalent strict regulations are being required.

Prior Art Documents

Patent Documents

[0004] [Patent Document 1] Special Publication No. 2022-534589 [Overview of the project] [Problems that the invention aims to solve]

[0005] This invention was made against this backdrop and provides a technique for lowering the lower limit of the feed flow rate of a distillation column through operational changes without requiring any modifications to the equipment design. [Means for solving the problem]

[0006] The present invention relates to a method for operating a distillation column, A step of adjusting the fluid to be treated supplied to the distillation column to a supply flow rate during turndown operation, which is lower than the supply flow rate during design operation, With respect to the distillation column to which the fluid to be treated is supplied at the supply flow rate during the turndown operation, the properties of the bottom product, which is the product or intermediate product flowing out from the bottom of the column, are made heavier than in the design operation while satisfying the design requirements for the bottom product, thereby increasing the turndown reflux flow rate and turndown reboiler flow rate compared to the assumed reflux flow rate and assumed reboiler flow rate that would be assumed if the heavier treatment were not performed during the turndown operation, and thereby increasing the gas-liquid load inside the column. Even when the process of raising the gas-liquid load inside the column is carried out, the process of verifying that the properties of the product or intermediate product (top product) flowing out from the top of the distillation column meet the design requirements of the top product, It is characterized by including. Alternatively, the present invention relates to a method for operating a distillation column, A step of adjusting the fluid to be treated supplied to the distillation column to a supply flow rate during turndown operation, which is lower than the supply flow rate during design operation, With respect to the distillation column to which the fluid to be treated is supplied at the supply flow rate during the turndown operation, the properties of the top product, which is the product or intermediate product flowing out from the top of the column, are made heavier than in the design operation while satisfying the design requirements for the top product, thereby increasing the turndown reflux flow rate and turndown reboiler flow rate compared to the assumed reflux flow rate and assumed reboiler flow rate that would be assumed if the heavier treatment were not performed during the turndown operation, and thereby increasing the gas-liquid load inside the column. Even when the process of increasing the gas-liquid load inside the column is carried out, the process of verifying that the properties of the product or intermediate product (bottom product) flowing out from the bottom of the distillation column meet the design requirements of the bottom product, It is characterized by including.

[0007] The operation method of the distillation column may have the following characteristics. (a) The distillation column is provided in the gas treatment device. (b) The turndown operation shall be performed when the distillation column is started up, or the turndown operation shall be performed in response to a change in the operating conditions of the equipment located upstream of the distillation column. (c) The supply flow rate during the turndown operation shall be within the range of 10 to 40% of the supply flow rate during the design operation. (d) The supply flow rate of the fluid to be treated during the turndown operation shall be less than or equal to the lower limit of the supply flow rate determined by the structural factor according to the structure of the distillation column. [Effects of the Invention]

[0008] This invention increases the turndown reflux flow rate and turndown reboiler flow rate compared to when the properties of the bottom or top products are not adjusted, by making the properties of the bottom product and the top product heavier than in the design operation during turndown operation. As a result, the gas-liquid load inside the tower is increased, and the lower limit of the supply flow rate during turndown operation can be lowered. [Brief explanation of the drawing]

[0009] [Figure 1] This is a block flow diagram showing the general configuration of an LNG plant. [Figure 2] This is a diagram showing the configuration of the heavy component recovery equipment and distillation equipment installed in the aforementioned LNG plant. [Figure 3] This flow scheme highlights the key changes in operating conditions of the distillation equipment during turndown operation. [Figure 4] This is a diagram (tray capacity graph) showing the change in the gas-liquid load inside the tower before and after applying the stabilizer operation method of this embodiment. [Figure 5] This flow scheme highlights the key operational changes made to the heavy component recovery equipment during turndown operation. [Figure 6] This is a diagram showing another example of a heavy component recovery system (an example using a gas expander). [Modes for carrying out the invention]

[0010] As an example of a gas treatment apparatus equipped with a distillation column to which the operating method of the present invention is applied, we will describe an example of an LNG plant for liquefaction treatment to produce LNG from natural gas (NG). Figure 1 shows a schematic representation of an LNG plant using blocks. Each block contains numerous pieces of equipment and devices, including static equipment such as towers, tanks, and heat exchangers, dynamic equipment such as pumps, compressors, and turbines, and piping that connects these static and dynamic components.

[0011] In the LNG plant of the present embodiment, the NG, which is the raw material fluid supplied from the wellhead, is subjected to pretreatment to remove various impurities after the liquid (condensate) contained in the NG is separated by the gas-liquid separation facility 11. The condensate separated by the gas-liquid separation facility 11 is stored in the condensate tank 18 for shipment. The NG after gas-liquid separation is subjected to removal of carbon dioxide and hydrogen sulfide, which are acid gases that may solidify in the LNG during liquefaction, by the acid gas removal facility 12. The acid gas removal facility 12 may be constituted by, for example, an acid gas absorption facility including an absorption tower for countercurrently contacting an absorption liquid that absorbs the acid gas with natural gas and a regeneration tower for the absorption liquid. Or the acid gas removal facility 12 may be constituted by, for example, a DDR type zeolite membrane made of zeolite or a membrane separation facility provided with a gas separation membrane composed of a large number of hollow fiber membranes in the main body.

[0012] The NG from which the acid gas has been removed is further subjected to removal of moisture and mercury by the moisture and mercury removal facility 13. For example, the moisture and mercury removal facility 13 includes a moisture adsorption tower filled with an adsorbent that adsorbs moisture and a mercury adsorption tower filled with a mercury adsorbent that adsorbs mercury. Note that the order of removing the impurities is not limited to the above example. For example, the mercury adsorption tower may be arranged in front of the acid gas removal facility 12.

[0013] Next, the natural gas from which the impurities have been removed is cooled and distilled and separated by the heavy component recovery facility 14 into light hydrocarbons mainly composed of methane (hereinafter, also simply referred to as "light components") and heavy hydrocarbons, which are liquid hydrocarbon components having 5 or more carbon atoms (hereinafter, also referred to as "heavy components"). The detailed configuration of the heavy component recovery facility 14 will be described later.

[0014] In addition, the heavy components separated by the heavy component recovery facility 14 contain somewhat ethane, propane, and butane. In the distillation facility 16, ethane, propane, butane, and condensate, which is a heavy hydrocarbon with C5+ or higher, are precisely separated by distillation. Most of the light component liquid of ethane, propane, and butane separated by distillation in the aforementioned distillation facility 16 is returned as a reflux liquid to the top of the scrub column 23 (to be described later) of the heavy component recovery facility 14. At the top of the scrub column 23 of the heavy component recovery facility 14 where the reflux liquid of ethane, propane, and butane is returned, the excess C5+ components contained in the gas composition are brought into direct contact with the supercooled reflux liquid (ethane, propane, butane), and the heavy components of the C5+ components contained in the gas components at the top of the scrub column 23 mainly composed of light components are absorbed to satisfy the design requirement specifications. In addition, the remaining mixed liquid of ethane, propane, and butane that was separated by distillation in the aforementioned distillation facility 16 and not returned to the heavy component recovery facility 14 is sent to the liquefaction facility 15 together with the light components mainly composed of methane separated by distillation in the heavy component recovery facility 14. On the other hand, the condensate is sent to the condensate tank.

[0015] [[ID=⑥]]Here, a part of the light components separated by distillation in the distillation facility 16, for example, propane, is stored in the refrigerant component tank 19. The propane stored in the refrigerant component tank 19 is replenished as a refrigerant component of the external refrigerant used in the external cooler 26 of the liquefaction facility 15. The liquid transfer line from the distillation facility 16 to the refrigerant component tank 16 and the refrigerant replenishment line from the refrigerant component tank 16 to the external refrigerant cooler 26 of the liquefaction facility 15 shown by the dashed arrows in FIG. 1 are not always used, but are used as needed.

[0016] It should be noted that in the translation of the above content, the numbering of some tags may not have a direct logical connection with the translated text content, but they are preserved as required. Also, for the Chinese characters in the original text, some may be related to Japanese content, and the translation is carried out according to the overall context. If there are any inaccuracies, please adjust according to the actual situation.The light components from which the heavy components have been removed in the heavy component recovery facility 14, along with ethane, propane, and butane that have been combined from the distillation facility 16, are cooled and liquefied in the liquefaction facility 15 to become liquefied natural gas (LNG). The liquefaction facility 15 is equipped with a main cryogenic heat exchanger (MCHE) for liquefying the light components using a main refrigerant (a mixed refrigerant consisting of methane, ethane, propane, butane, isopentane, and nitrogen, etc.) or a nitrogen-only refrigerant. The main cryogenic heat exchanger (MCHE) may be composed of a spiral-wound type or a cold-box type heat exchanger.

[0017] Some LNG systems are designed to increase production by vaporizing a portion of the LNG (end flush) to obtain cryogenic heat at around -160°C. Whether or not an end flush is performed, the liquid after leaving the main cryogenic heat exchanger is sent to the LNG tank 17 for shipment.

[0018] Furthermore, a portion of the liquid (condensate) separated from natural gas in the gas-liquid separation equipment 11 is subjected to a process to remove light components in a condensate stabilizer (not shown) for recovering heavy components, and then stored together with the condensate obtained in the distillation equipment 16 in a condensate tank 18 for shipment.

[0019] In the LNG plant with the above configuration, as previously described, the heavy component recovery equipment 14 distills and separates natural gas into a light component mainly composed of methane and a heavy component with 5 or more carbon atoms. The distillation equipment 16 further distills and separates the light components of ethane, propane, and butane contained in the heavy component from condensate, which is a heavy hydrocarbon with 5 or more carbon atoms. To perform these distillation separations, the heavy component recovery equipment 14 and the distillation equipment 16 are each equipped with distillation columns (scrub columns 23 and stabilizers 32).

[0020] As explained in the background technology section, these distillation columns have a lower limit on the feed flow rate, which is determined by the mechanical structural factors of the distillation column. Examples of structural factors include the column diameter, column height, and internal structures such as the tray structure. This embodiment provides a technology to lower the lower limit on the feed flow rate by improving the operating method of each distillation column installed in the heavy component recovery equipment 14 and the distillation equipment 16. Before explaining the details of the improvements to the operating method, an example of the configuration of the heavy component recovery equipment 14 and the distillation equipment 16 will be described with reference to Figure 2.

[0021] There are two methods for separating methane from heavy components: the self-refrigerating separation method and the distillation separation method. The self-refrigerating separation method uses a self-refrigerant obtained by reducing the pressure of NG to separate the heavy components. The distillation separation method, on the other hand, separates the heavy components using an external refrigerant such as propane or a mixed refrigerant without reducing the pressure of NG. Figure 2 shows the heavy component recovery equipment 14 of the self-refrigerating heavy component recovery method.

[0022] The heavy component recovery equipment 14 includes an NG supply line 201 to which NG that has undergone a process to remove impurities upstream is supplied, a cold box 21 for cooling the NG supplied from the NG supply line 201, a feed separator 22 for gas-liquid separation of the NG that has been cooled in the cold box 21 and partially liquefied, and a scrub column 23, which is a distillation column of this embodiment, for distilling the NG and separating it into a light component mainly composed of methane and a heavy component with 5 or more carbon atoms.

[0023] The NG supplied from the NG supply line 201 is cooled in the cold box 21 and separated into gas and liquid by the feed separator 22. The separated gas is expanded under reduced pressure and then supplied to the scrub column 23 as a gas-liquid mixed fluid. Similarly, the liquid separated from the gas and liquid by the feed separator 22 is also expanded under reduced pressure and then supplied to the scrub column 23 as a gas-liquid mixed fluid. The gas-liquid mixed fluid supplied to the scrub column 23 via the feed separator 22 corresponds to the fluid being processed by the scrub column 23.

[0024] Furthermore, a portion of the NG supplied via the NG supply line 201 is extracted as stripping gas from the stripping gas line 203 before reaching the cold box 21 and supplied to the bottom of the scrub column 23. The stripping gas plays a role in regasifying the light component gases and liquefying the heavy component gases in the self-gas by directly bringing the cold liquid of the light components that has descended to the bottom of the scrub column 23 into gas-liquid contact with the higher-temperature self-gas (stripping gas). For this reason, the scrub column 23 may be equipped with a reboiler instead of a stripping gas supply mechanism.

[0025] The scrub column 23 recovers the light components, mainly methane, as a gas by distilling the supplied gas-liquid mixture of NG, and sends it to the cold box 21 of the downstream equipment via the gas supply line 202 located at the top of the column. In addition, the heavy components (C5+) in liquid form are separated from the bottom of the column. These heavy components are sent to the distillation equipment 16, which is the downstream equipment, as the bottom product, an intermediate product extracted from the bottom of the scrub column 23.

[0026] The light components, mainly methane, that flow out from the scrub column 23 are used as a refrigerant for the cold box 21, then pressurized by the LNG feed gas compressor 24, and further cooled by the LNG feed gas compressor discharge air cooler 25 and the external refrigerant cooler 26. After cooling, the light components are sent to the liquefaction plant 15 as an intermediate product (top product) extracted from the top of the scrub column 23.

[0027] The distillation apparatus 16 includes a stabilizer 32, which is a distillation column of this embodiment, that precisely distills and separates the heavy components separated in the scrub column 23 into light gases containing ethane, propane, butane, and nitrogen, and condensate with C5+ or higher. The heavy components that flow out from the bottom of the scrub column 23 are heated in the stabilizer preheating heat exchanger 31 and then supplied to the stabilizer 32. The stabilizer 32 distills the heavy components supplied from the scrub column 23, separating them into light components such as ethane, propane, butane, and nitrogen, and condensate with C5+ or higher. The stabilizer 32 is equipped with a stabilizer reboiler 34 that heats the bottom liquid and returns it to the stabilizer 32.

[0028] The gas extracted from the top of the stabilizer 32 is cooled in an air-cooled (or water-cooled) stabilizer condenser 331 to become a gas-liquid mixture, which is then separated into gas and liquid by a stabilizer flux separator 332. The liquid extracted from the stabilizer flux separator 332 is resupplied as flux to the top of the stabilizer 32 by a stabilizer flux pump 333.

[0029] The gas extracted from the stabilizer-reflux separator 332 is cooled by an external refrigerant (e.g., propane refrigerant) in the LPG production cooler 38 to become a gas-liquid mixture, which is then separated into gas and liquid in the LPG separator 36. The liquid mixture of ethane, propane, and butane extracted from the LPG separator 36 is transported by the scrubber-reflux pump 37, and a portion of it is supplied to the top of the scrub column 23 via the LPG recycling line 204. The remaining mixture of ethane, propane, and butane that is not used for LPG recycling is extracted from the top of the stabilizer 32 as an intermediate product (top product), and at the outlet of the LNG feed gas compressor discharge air cooler 25, it merges with the main fluid of light components, mainly methane, from the scrub column 23 side, before being sent to the liquefaction facility 15.

[0030] Furthermore, the light gas, mainly composed of methane and nitrogen, extracted from the LPG separator 36 is sent to a BOG (Boil Off Gas) compressor (not shown). The BOG compressor pressurizes the end flush gas, the boil-off gas (BOG) evaporated from LNG in the LNG tank 17, and the light gas separated in the LPG separator 36, and supplies them as fuel gas to fuel gas consumers such as gas turbines or boilers. Alternatively, instead of being sent to the BOG compressor and used as fuel gas, the light gas, mainly composed of methane and nitrogen, extracted from the LPG separator 36 can also be pressurized by a newly installed light gas booster compressor (not shown here), merged at the outlet of the LNG feed gas compressor discharge air cooler 25, and recovered as LNG feed gas in the product.

[0031] Meanwhile, the condensate with a rating of C5+ or higher extracted from the bottom of the stabilizer 32 is used in the heat exchanger 31 as a heat transfer medium to heat the heavy components supplied to the stabilizer 32, and then cooled in the stabilizer product air cooler 35. After cooling, the condensate is sent to the condensate tank 18 as the product extracted from the bottom of the stabilizer 32 (bottom product).

[0032] In an LNG plant equipped with the heavy component recovery equipment 14 and distillation equipment 16 as described above, during normal operation, NG is supplied in an amount commensurate with the amount of LNG that can be produced. Therefore, the scrub column 23 of the heavy component recovery equipment 14 and the stabilizer 32 of the distillation equipment 16 are supplied with an appropriate amount of fluid to be processed, corresponding to the supply flow rate of the NG (NG after impurity removal to the scrub column 23, and heavy components separated by distillation in the scrub column 23 to the stabilizer 32).

[0033] In this application, operation under the conditions specified at the time of design is referred to as "design operation," and the NG supply flow rate during design operation is set to 100%. In this case, operation at an NG supply flow rate within the range from the lower limit determined by the mechanical structural factors of the scrub column 23 and the stabilizer 32 of the distillation equipment 16 up to 100% is referred to as "normal operation."

[0034] Generally, a distillation column used for distilling a fluid to be treated has a defined operating range that allows for stable operation. This operating range is determined by the mechanical structural factors of the distillation column, based on the column diameter, height, and internal structure such as trays and packing materials installed within the column. Limit values ​​are set within this operating range based on various factors, such as jet flooding, blowing (liquid leakage), and the limit of the cap flow velocity if caps are provided on the trays. The operation of the distillation column is then carried out so that the liquid flow rate descending to each tray stage and the gas flow rate ascending the tray stage remain within the operating range determined during the design phase (for example, the range shown in gray in Figures 4(a) and (b)).

[0035] In the heavy component recovery equipment 14 and distillation equipment 16 of this example, the scrub column 23 and stabilizer 32, as explained in Figure 2, have defined operating ranges. These scrub column 23 and stabilizer 32 are designed so that, during normal operation, the liquid flow rate and gas flow rate within the column remain within the operating range. Therefore, even if the NG supply flow rate is reduced by, for example, 10% to respond to temporary operational fluctuations in the LNG plant, the scrub column 23 and stabilizer 32 are designed to continue stable distillation operation within their respective operating ranges.

[0036] On the other hand, as explained in the background technology section, during startup, it takes time for the properties of each intermediate product and product to meet the design requirements of the scrub column 23 and stabilizer 32. During this startup, the NG supply flow rate may be lower than the lower limit (turndown constraint) based on the mechanical structural factors of the scrub column 23 and stabilizer 32. As a result, the turndown constraint becomes a bottleneck, making it difficult to meet product specifications, and off-spec products must be continuously supplied to the flare stack. To address these challenges, the distillation column operation method according to this embodiment allows for a further reduction in the NG supply flow rate when starting up the LNG plant, while satisfying the turndown constraint. The specific details of the operation method of the distillation column in this embodiment will be explained below, with reference to Figures 3 and 4 in addition to Figure 2.

[0037] In Figure 2, it is assumed that NG is supplied to the scrub column 23 via the NG supply line 201 at the supply flow rate specified in the design operation. In the following explanation, the supply flow rate of NG in this design operation will be assumed to be 100%. In the design operation, where NG of a predetermined composition is supplied at 100% of the supply flow rate, the scrub column 23 and stabilizer 32 are operated to meet the design requirements. The design requirements include tolerances for the flow rate, composition, and distillation properties of each bottom product and each top product obtained by distillation separation in the scrub column 23 and stabilizer 32.

[0038] For manufacturing top and bottom products that meet the design requirements, the operating variables for the scrub column 23 include an operating pressure range of 20 to 50 barg and a top operating temperature range of -25 to -70°C. In particular, the flow rate of LPG recycled gas supplied via the LPG recycling line 204 is greatly influenced by the concentration of heavy C5+ components, especially BTEX (Benzene, Toluene, Ethylbenzene, and Xylene), contained in the gas-liquid mixture (hereinafter also referred to as "feed gas") supplied via the feed separator 22. The supply flow rate of the stripping gas supplied via the stripping gas line 203 can be exemplified as being in the range of 3 to 15 mol% of the feed gas flow rate. These LPG recycled gas flow rates and stripping gas supply flow rates have a significant impact on the liquid flow rate and gas flow rate within the scrub column 23.

[0039] Furthermore, examples of operating variables for manufacturing top and bottom products that meet the design requirements for the stabilizer 32 include an operating pressure range of 5 to 20 barg and a top operating temperature range of 10 to 80°C. In particular, the flux flow rate supplied from the stabilizer flux separator 332 to the top of the stabilizer 32, and the reboiler flow rate to which the bottom liquid heated in the stabilizer reboiler 34 is returned to the stabilizer 32, have a significant impact on the liquid flow rate and gas flow rate within the stabilizer 32.

[0040] In particular, in the example shown in Figure 2, the stabilizer 32 is operated under normal conditions so that the reid vapor pressure (RVP) of the condensate (a mixture of condensate separated into gas and liquid in the gas-liquid separation equipment 11 and treated in the stabilizer to remove light components) in the condensate tank meets the design requirements. Here, 9 psia (pounds square inch absolute) is given as an example of the design requirements. Generally, the RVP can be in the range of 8 to 12 psia.

[0041] Next, we will explain the operating method during turndown operation, which is performed when the NG supply flow rate is reduced compared to the design operation, when starting up the LNG plant. There are no particular limitations on the NG supply flow rate as long as it is in a range higher than the lower limit determined by mechanical structural constraints (turndown constraints), but we will illustrate the case where the supply flow rate is lower than that during turndown operation and is performed only by operating operations that do not involve modifications to the equipment. An example of the supply flow rate during turndown operation is to set it to a value within the range of 10 to 40% of the design flow rate. Figure 3 shows the case where the supply flow rate of NG during turndown operation, supplied via the NG supply line 201, is set to 20%, which is within the range of 10 to 40% of the design operation.

[0042] First, we will describe a method of starting the LNG plant and supplying NG to the scrub column 23 and stabilizer 32, which are distillation columns, at a supply flow rate below the lower limit due to turndown constraints (in this example, 20% of the design operation supply flow rate). When starting the LNG plant, the supply flow rate of NG is gradually increased after the supply is started, and once it reaches 20% of the design operation supply flow rate (hereinafter also referred to as the "design flow rate"), that supply flow rate is maintained. Through this operational adjustment, the supply flow rate of heavy components supplied to the stabilizer 32 of the distillation equipment 16 will also be 20% of the design flow rate (a process of adjusting to the supply flow rate during turndown operation). At this time, assuming that the design operation's flux flow rate and reboiler flow rate in stabilizer 32 are 20% of the design flow rate, it can be seen that the liquid flow rate and gas flow rate within stabilizer 32 fall outside the appropriate operating range, as shown in the operating point in Figure 4(a) of the tray capacity graph.

[0043] Therefore, in this embodiment, in the stabilizer 32, where the heavy component (fluid to be treated) is supplied at the supply flow rate during the turndown operation described above, the properties of the condensate flowing out from the bottom of the tower are set to be heavier than in the design operation while still meeting the design requirements. Specifically, the condensate is made heavier so that the RVP in the condensate tank is 8 psia or less, which is within the range of the design requirements.

[0044] The setting for increasing the condensate concentration includes adjusting the NG supply flow rate to 20% of the design flow rate, followed by increasing the stabilizer 32's flux flow rate and reboiler flow rate. The flux flow rate and reboiler flow rate at this time (hereinafter referred to as "turndown flux flow rate and turndown reboiler flow rate") are higher than the expected flux flow rate and reboiler flow rate (hereinafter referred to as "expected flux flow rate and expected reboiler flow rate") when the NG supply flow rate is adjusted to 20% of the design flow rate. By performing these operational adjustments, both the liquid flow rate and gas flow rate (hereinafter also referred to as "in-stack gas-liquid load") flowing through the stabilizer 32 increase (the process of increasing the in-stack gas-liquid load). In Figure 3, equipment / lines whose operating conditions have changed significantly by increasing the turndown flux flow rate and turndown reboiler flow rate compared to the expected flux flow rate and expected reboiler flow rate are shown with thick lines.

[0045] Here, Figure 4(b) shows a tray capacity graph when the condensate RVP is reduced to 8 psia. It represents the operating range within the stabilizer 32 after increasing the gas-liquid load inside the tower, and shows the operating points corresponding to the liquid flow rate and gas flow rate in 20% turndown operation. As shown in the figure, by increasing each flow rate up to the turndown reflector flow rate and the turndown reboiler flow rate, it becomes possible to operate the stabilizer 32 appropriately within the operating range even during turndown operation. Thus, the operating range of the stabilizer 32 changes significantly depending on whether the condensate RVP is 8 psia or 9 psia. Therefore, Figures 4(a) and (b) illustrate a case in which, without changing the mechanical design, the design requirements are met, and operation is possible within the operating range even though it is outside the operating range at 9 psia (Figure 4(a)), by reducing it to 8 psia (Figure 4(b)).

[0046] Next, even when increasing the flow rates to the turndown reflux flow rate and the turndown reboiler flow rate to raise the gas-liquid load inside the tower, it is necessary to verify that the properties of the top product flowing out from the top of the stabilizer 32 meet the design requirements (process for verifying that the top product meets the design requirements). In order to verify that the design requirements are met, it is necessary to actually measure and analyze the flow rates and properties of the top and bottom products.

[0047] In the distillation apparatus 16, the mixture of ethane, propane, and butane, after gas-liquid separation by the LPG separator 36, is used as the flux for the scrub column 23.

[0048] The above explanation uses the operation method of the stabilizer 32 during turndown operation as an example, but the same concept can be applied to the scrub column 23 of the heavy component recovery equipment 14. That is, as shown in Figure 5, the NG supplied to the stabilizer 32 via the NG supply line 201 is accepted entirely into the scrub column 23 even if the supply flow rate is less than the design supply flow rate and below the lower limit of turndown operation determined by mechanical structural constraints (20% of the design operation in this example) (a process of adjusting the supply flow rate).

[0049] Next, the properties of the heavy components (C5+) in the gas flowing out from the top of the scrub column 23 are set to be heavier than the design operation while still meeting the design requirements. In the scrub column 23, the liquid flow rate of LPG recycling supplied via the LPG recycling line 204 must also be increased. Therefore, if there is liquid in the stream 205 that handles the surplus LPG liquid flow rate, the required flow rate is supplied to the scrub column 23 from the liquid in stream 205 via the LPG recycling line 204. In addition, the supply flow rate of the stripping gas supplied via the stripping gas line 203 is increased above the assumed flow rate (a process to raise the gas-liquid load inside the column). These LPG recycling flow rates and stripping gas supply flow rates correspond to the "turndown reflux flow rate and turndown reboiler flow rate" when performing the process of raising the gas-liquid load inside the column in the scrub column 23. If the amount of LPG liquid is still insufficient, it is also possible to replenish with LPG liquid mainly composed of butane from an external source to secure the reflux flow rate (see the dashed arrow line shown in Figure 5). Furthermore, even when performing the process of increasing the gas-liquid load inside the column, it is necessary to actually measure and analyze the flow rate and properties of the top product flowing out from the top of the scrub column 23 in order to verify that the properties of the top product flowing out from the top of the scrub column 23 meet the design requirements (process to verify that the design requirements of the top product are met).

[0050] The operation method of the distillation column according to this embodiment has the following effects. During turndown operation, the properties of the bottom product of the stabilizer 32 and scrub column 23 are made heavier than in the design operation, and the turndown reflux flow rate and turndown reboiler flow rate are increased compared to when the properties of the bottom product are not adjusted. As a result, it becomes possible to increase the gas-liquid load inside the column of the stabilizer 32 and scrub column 23. These operational adjustments make it possible to perform turndown operation at a supply flow rate (for example, 20% of the design operation) below the lower limit of NG turndown, which would have been impossible to implement in the past because it would have fallen outside the appropriate operating range of the stabilizer 32 and scrub column 23 and would have made it impossible to meet the design requirements.

[0051] For example, if the feed gas supply flow rate is limited to a level below the lower limit during turndown operation, which is determined by the mechanical structural constraints of the scrub column 23 and stabilizer 32, off-spec products that do not meet the design requirements will be generated at the top and bottom of the column, and these must be burned by flaring. This results in an increase in CO2 emissions (GHG) and increased product loss. In contrast, this embodiment raises the lower limit of turndown operation by making the product heavier within the range that meets the design requirements, thereby reducing CO2 emissions and product loss.

[0052] In the embodiments shown in Figures 3 and 5, a method was described in which the gas-liquid load inside the column is increased by adjusting the turndown reflux flow rate and turndown reboiler flow rate, focusing on the properties of the bottom product of the stabilizer 32 and scrub column 23, so that the bottom product becomes heavier while satisfying the design requirements. However, the object of focus for satisfying the design requirements while adjusting the turndown reflux flow rate and turndown reboiler flow rate is not limited to the bottom product.

[0053] For example, the supply flow rate of the fluid to be treated to the stabilizer 32 and scrub column 23 may be adjusted to the supply flow rate during turndown operation (step of adjusting the supply flow rate), and then, focusing on the top product flowing out from the top of the column, the turndown reflux flow rate and turndown reboiler flow rate may be increased to make the top product heavier than in the design operation while still meeting the design requirements (step of increasing the gas-liquid load inside the column). After that, it is verified that the properties of the bottom product flowing out from the bottom of the stabilizer 32 meet the design requirements (step of verifying that the bottom product meets the design requirements).

[0054] Furthermore, the reasons for performing turndown operation are not limited to reducing large amounts of hydrocarbons by burning off-spec gas in the flare stack during the period until the design requirements of the liquefaction equipment 15 are met at the start of operation of the LNG plant. The operation method of the distillation column according to the above embodiment may also be applied when turndown operation becomes necessary from normal operation due to changes in the operating conditions of upstream equipment, such as equipment troubles other than the distillation equipment 16 and the heavy component recovery equipment 14, or a decrease in the supply flow rate of raw material NG from the wellhead.

[0055] The above-described operating method, which increases the turndown reflector flow rate and turndown reboiler flow rate during turndown operation, is not limited to its application to the heavy component recovery equipment 14 with the configuration described using Figure 2, etc. For example, this operating method may also be applied to the heavy component recovery equipment 14a, which uses the expander 271 shown in Figure 6 to obtain a lower temperature self-refrigerant. The heavy component recovery equipment 14a shown in Figure 6 is configured to expand a portion of the gas after gas-liquid separation by the feed separator 22 under reduced pressure in the expander 271 and supply it to the scrub column 23 as a low-temperature gas-liquid mixture. The remaining gas that is not supplied to the expander 271 is supercooled in the scrubber reflector cold box 211, then expanded under reduced pressure in a pressure reducing valve (not shown), and supplied as a reflector liquid to the top of the scrub column 23. Furthermore, in Figure 6, reference numeral 28 indicates a scrubber reboiler that heats the bottom liquid of the scrub column 23, and reference numeral 272 indicates a compounder that recovers the power to rotate the expander 271 and compresses the light components.

[0056] Furthermore, this operating method is not limited to applications to the stabilizer 32 and scrub column 23. For example, instead of the stabilizer 32, multiple rectification columns (deethanolizer, depropanizer, debutanizer) may be provided to sequentially distillate and separate ethane, propane, and butane contained in heavy components of C5+ or higher. The operating method of the distillation column according to this embodiment may also be applied to these rectification columns.

[0057] Furthermore, the plants to which the distillation column operation method of this embodiment can be applied are not limited to LNG plants (gas processing facilities). The distillation column operation method of this embodiment can also be applied to distillation columns installed in various plants, such as NGL (ethane, propane, butane) recovery units, petroleum refining plants that perform distillation and desulfurization of various intermediate products from crude oil, and chemical plants that produce petrochemical products, intermediate chemicals, or polymers from ethylene, etc. [Explanation of symbols]

[0058] 11 Gas-liquid separation equipment 12. Acid Gas Removal Equipment 13 Moisture and mercury removal equipment 14, 14a Heavy component recovery equipment 15 Liquefaction equipment 16 Distillation equipment 17 LNG tanks 18 Condensate Tanks 19 Refrigerant component tank 201 NG supply line 202 Air supply line 203 Stripping Gas Line 204 LPG Recycling Line 21 Cold Box 211 Scrubber Reflux Cold Box 22 Feed Separator 23 Scrub Columns 24 LNG Feed Gas Compressors 25 LNG Feed Gas Compressor Discharge Air Cooler 26. External refrigerant cooler 271 Expander 272 Compounder 28 Scrubber Reboiler 31 Stabilizer preheating heat exchanger 32 Stabilizer 331 Stabilizer Capacitor 332 Stabilizer Reflux Separator 333 Stabilizer Reflux Pump 34 Stabilizer Reboiler 35 Stabilizer Products Air Cooler 36 LPG Separator 37. Scrubber Reflux Pump 38 LPG Production Cooler

Claims

1. A method for operating a distillation column, A step of adjusting the fluid to be treated supplied to the distillation column to a supply flow rate during turndown operation, which is lower than the supply flow rate during design operation, With respect to the distillation column to which the fluid to be treated is supplied at the supply flow rate during the turndown operation, the process of increasing the turndown reflux flow rate and the turndown reboiler flow rate to make the properties of the bottom product, which is the product or intermediate product flowing out from the bottom of the column heavier than in the design operation while satisfying the design requirements of the bottom product, thereby increasing the gas-liquid load inside the column, Even when the process of raising the gas-liquid load inside the column is carried out, the process of verifying that the properties of the product or intermediate product (top product) flowing out from the top of the distillation column meet the design requirements of the top product, A method for operating a distillation column, characterized by including [a specific element].

2. A method for operating a distillation column, A step of adjusting the fluid to be treated supplied to the distillation column to a supply flow rate during turndown operation, which is lower than the supply flow rate during design operation, With respect to the distillation column, which is supplied with the fluid to be treated at the supply flow rate during the turndown operation, the process of increasing the turndown reflux flow rate and the turndown reboiler flow rate to make the properties of the top product, which is the product or intermediate product flowing out from the top of the column heavier than in the design operation while meeting the design requirements for the top product, thereby increasing the gas-liquid load inside the column, Even when the process of increasing the gas-liquid load inside the column is carried out, the process of verifying that the properties of the product or intermediate product (bottom product) flowing out from the bottom of the distillation column meet the design requirements of the bottom product, A method for operating a distillation column, characterized by including [a specific element].

3. The method for operating a distillation column according to claim 1 or 2, characterized in that the distillation column is provided in a gas treatment device.

4. The method for operating a distillation column according to claim 1 or 2, characterized in that the turndown operation is performed when the operation of the distillation column is started.

5. The method for operating a distillation column according to claim 1 or 2, characterized in that the turndown operation is performed in conjunction with a change in the operating conditions of equipment located upstream of the distillation column.

6. The method for operating a distillation column according to claim 1 or 2, characterized in that the supply flow rate during the turndown operation is within the range of 10 to 40% of the supply flow rate during the design operation.

7. The method for operating a distillation column according to claim 1 or 2, characterized in that the supply flow rate of the fluid to be treated during the turndown operation is below the lower limit of the supply flow rate determined by a structural factor corresponding to the structure of the distillation column.

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